1. Field of the Invention
The present invention relates to a piezoelectric ceramic and more particularly relates to a bismuth-layer compound which may be utilized in the field of a resonator, a high temperature sensor and the likes.
2. Description of the Related Art
The piezoelectric ceramic is widely used in the field of electronic appliance including resonator, filter and others as well as in the products including sensor or actuator in which electric charge and displacement are utilized.
The conventional piezoelectric ceramic has been generally ferroelectrics having the perovskite structure such as PbZrO3xe2x80x94PbTiO3 solid solution (called as xe2x80x9cPZTxe2x80x9d hereinafter) of pyromidal quadratic system or of rhombic system and PbTiO3 (called as xe2x80x9cPTxe2x80x9d hereinafter) of pyromidal quadratic system. These materials may have various piezoelectric properties with sub-components being added thereto.
However, many of the piezoelectric ceramics of PZT group or of PT group have generally the Curie point of 200 to 400xc2x0 C. and will be reduced to paraelectrics at a higher temperature and lose the piezoelectric properties. Such ceramic is, therefore, not adapted for use, for example, as an atomic reactor control sensor under a high temperature. Further the piezoelectric ceramic of PZT system or of PT system contains lead oxide (PbO) of 60 to 70 wt %. The piezoelectric ceramic is, therefore, undesirable in view of ecology as well as public pollution.
It has been desired that the piezoelectric ceramic having a property of higher Curie point and containing no lead oxide will be realized.
In response to the desire, the (1xe2x88x92x)(Bi1/2Na1/2) TiO3xe2x88x92xNaNbO3 solid solution of the same perovskite structure is disclosed (JP-A-9-100156) as a piezoelectric ceramic containing no lead. The piezoelectric ceramic is, however, of the Curie point below the temperature 370xc2x0 C. The piezoelectric ceramic, therefore, can not be used as a sensor for controlling the atomic reactor of higher temperature.
Further the bismuth-layer compound is known as a piezoelectric ceramic having a Curie point above the temperature 400xc2x0 C. and containing no lead. The bismuth-layer compound is thermally stabilized due to the high Curie point when it is once polarization-treated and comes to have an excellent property as a high temperature-sensor. However, since the polarization itself is difficult, the polarization is liable to end in incomplete result. Thus the compound has the difficulty for obtaining a piezoelectric property which is sufficient to be used in connection with the electronic appliance.
On the other hand, in case of the resonator which is used as an inductor, the piezoelectric ceramic is required to have one of the important piezoelectric properties, that is, a large Qm (coefficient of mechanical quality) or a large Qmax (Q=maximum value of tan xcex8, xcex8:phase) between resonance frequency and anti-resonance frequency. However, there has not been such a piezoelectric ceramic of bismuth-layer compound reported to have a sufficient piezoelectric property so as to be effectively used in connection with the resonator.
The present invention has been made in view of such circumstances. It is an object of the invention to provide a piezoelectric ceramic containing no lead and having a high Curie point and excellent piezoelectric properties, especially a large Qmax.
For the purpose of attaining the object, the present invention provides a piezoelectric compound comprising a bismuth-layer compound having SrBi4Ti4O15-type crystal, the axial ratio c/a of the crystal lattice being in the range of 7.49 to 7.67.